The present invention relates to protection against corrosion for parts made of ceramic matrix composite (CMC) materials containing silicon, in particular CMC material parts having a matrix of silicon carbide (SiC). A particular but non-exclusive field of application for the invention is that of parts for the hot portions of gas turbines, such as the walls of combustion chambers, in particular for aeroengines.
For such gas turbines, the desire to improve efficiency and to reduce pollution emission leads to ever higher temperatures being envisaged in the combustion chamber.
Proposals have therefore been made to replace metal materials by CMC materials, in particular for the walls of combustion chambers. CMC materials are known for simultaneously presenting good mechanical properties enabling them to be used for structural elements, and for conserving these properties at high temperatures. CMC materials comprise fiber reinforcement made of refractory fibers, typically carbon fibers or ceramic fibers, which reinforcement is densified by a ceramic matrix, typically of SiC.
In the presence of a corrosive environment (oxidizing atmosphere, in particular in the presence of moisture and/or a saline atmosphere), a surface-retreat phenomenon is observed when using CMC materials having a an SiC matrix because the silica (SiO2) that forms by oxidation at the surface of the CMC material becomes volatilized.
It has been recommended that an environmental barrier should be formed at the surface of the CMC material. FIG. 1 is a highly diagrammatic section view of such a prior art barrier for a substrate 1 of CMC material having an SiC matrix. The anticorrosion function is provided by a layer 2 of a compound of the type comprising an aluminosilicate of an alkaline-earth metal, such as the compound BaO0.75.SrO0.25Al2O3(SiO2)2 commonly referred to by the abbreviation BSAS. A chemical barrier layer 3 is interposed between the layer of BSAS and the substrate in order to avoid chemical interactions between the BSAS and the SiC of the substrate. Typically the layer 3 comprises an association of mullite (in the majority) and of BSAS, the presence of BSAS reducing sensitivity to cracking compared with a layer of mullite on its own. A sub-layer 4 of silicon (Si) is formed on the substrate to facilitate bonding the chemical barrier layer 3. Documents US 2004/0151840, U.S. Pat. Nos. 6,866,897, and 6,787,195, amongst others, illustrate that prior art.
Such an environmental barrier has been found to have shortcomings.
When temperature reaches high values, typically above about 1300° C., retreat of the surface of the BSAS layer can be observed due to volatilization of the silica contained in said layer. This can be remedied by increasing the thickness of the layer of BSAS so as to obtain the desired lifetime. Another solution consists in providing the environmental barrier with an outer layer for thermal protection, in particular a layer of yttrium-stabilized zirconia (or “yttrified zirconia”), as described for example in documents U.S. Pat. Nos. 6,740,364, 6,558,814, 6,699,607, 6,607,852, EP 1 416 066, and EP 1 142 850. In addition, at these high temperatures, deterioration occurs by chemical interaction between the BSAS contained in the chemical barrier layer and the Si in the bond sub-layer on the substrate. To remedy that, a layer of mullite on its own can be interposed between the bond sub-layer made of Si and the chemical barrier layer made of mullite+BSAS, as described in particular in documents U.S. Pat. Nos. 6,759,151 and 6,733,908.
The Applicant has also observed that the chemical bond layer of Si is sensitive to the cracking that is caused by thermal shocks, which can lead to loss of cohesion of the environmental barrier.
The formation of a coating of mullite with a composition gradient on a substrate containing silicon, in particular a substrate of SiC is described in an article by Basu et al. “Formation of mullite coatings on silicon-based ceramics by chemical vapor deposition”, and in an article by Hou et al. “Structure and high temperature stability of compositionally graded CVD mullite coatings” published in “International Journal of Refractory Metals and Hard Metals”, Elsevier Publishers, Barking, GB, respectively in Vol. 16, No. 4-6, 1998, pp. 343-352, and Vol. 19, No. 4-6, July 2001, pp. 467-477. The mullite coating is formed by chemical vapor deposition (CVD). The composition of the coating varies from a phase that is rich in silica close to the substrate, to a phase that is rich in alumina close to the outer surface, the alumina-rich phase performing the anticorrosion function.